Method for mixing powdered metal and nanocarbon material, and method for manufacturing nanocarbon/metal composite material

ABSTRACT

A manufacturing method is provided to be used in place of a conventional mechanical alloying method. A powdered metal and a nanocarbon material are placed in an empty metal mill vessel containing no balls, and a mixture in which the powdered metal is coated with this nanocarbon material is obtained by shaking in three dimensions.

FIELD OF THE INVENTION

The present invention relates to a method for mixing a powdered metaland a nanocarbon material, and to a method for manufacturing ananocarbon/metal composite material.

BACKGROUND OF THE INVENTION

In recent years, special carbon fibers known as “carbon nanofibers” haveattracted attention. Carbon nanofibers have a configuration in whichsheets of carbon atoms arranged in the form of a hexagonal network arerolled up into a tubular form; such nanofibers have a diameter of 1.0 nmto 150 nm, and a length of a few micrometers to 100 μm. Since suchfibers have a nano-size diameter, they are referred to as “carbonnanofibers,” “carbon nanotubes,” or the like (such materials will becalled “nanocarbon materials” below).

These nanocarbon materials are reinforcing materials, and are alsomaterials with a good thermal conductivity. Accordingly, strength andthermal conductivity can be improved by mixing these materials withmetal materials.

In order to obtain the expected strength and thermal conductivity, it isessential that such nanocarbon materials be uniformly mixed with themetal materials.

One technique for uniformly mixing a nanocarbon material with a metalmaterial is mechanical alloying. This mechanical alloying method hasbeen proposed previously in Japanese Unexamined Patent Application No.2003-246613. In the mechanical alloying method, balls, a metal material,and a nanocarbon material are placed in a vessel, and the vessel isshaken or rotated. Consequently, the balls strike the nanocarbonmaterial whereby the nanocarbon material is broken up. As a result ofthe shaking or vibration, this broken-up nanocarbon material is broughtinto contact with the metal material and is strongly bonded to the metalmaterial.

However, since the carbon nanotubes are mechanically broken up andconverted into short fibers, no great improvement in thermal conductioncan be expected. Specifically, in the case of long fibers, these fibersconstitute passages for heat, so that a high thermal conductivity isobtained. However, in the case of short fibers, the thermal conductivityis small.

Thus, in conventional mechanical alloying methods, it has beenascertained that the desired thermal conductivity performance cannot besufficiently obtained.

Accordingly, there is a need for a manufacturing method to be used inplace of conventional mechanical alloying.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a method for mixing apowdered metal and a nanocarbon material is provided which comprises thesteps of preparing a ball mill, a specified amount of a powdered metal,and a specified amount of a nanocarbon material; placing theaforementioned powdered metal and nanocarbon material in the empty metalmill vessel containing no balls; and obtaining a mixture in which theaforementioned powdered metal is coated with the aforementionednanocarbon material by shaking the mill vessel in three dimensions usingthe aforementioned ball mill.

In the aforementioned mixing method, since no balls are placed in thevessel, there is no danger that the nanocarbon material will beexcessively broken up. Furthermore, the nanocarbon material can bebonded to the powdered metal by shaking the mill vessel in threedimensions. Accordingly, the nanocarbon material in the form of longfibers can be mixed with the powdered metal in a desirable manner.

The nanocarbon material that is prepared in the aforementionedpreparatory step is preferably a nanocarbon material that has beendispersed in advance by ultrasound. If the nanocarbon material is thusdispersed by ultrasound, and the dispersed nanocarbon material is placedin the mill vessel, the metal particles can be more uniformly coatedwith the nanocarbon material.

According to another aspect of the present invention, a method formanufacturing a nanocarbon/metal composite material is provided whichcomprises the steps of preparing a ball mill, a specified amount of apowdered metal, and a specified amount of a nanocarbon material; placingthe aforementioned powdered metal and nanocarbon material in the emptymetal mill vessel containing no balls; obtaining a mixture in which theaforementioned powdered metal is coated with the aforementionednanocarbon material by shaking the mill vessel in three dimensions usingthe aforementioned ball mill; and molding and sintering theaforementioned mixture to obtain a sintered body.

In the aforementioned method for manufacturing a nanocarbon/metalcomposite material, since no balls are placed in the vessel, there is nodanger that the nanocarbon material will be excessively broken up.Furthermore, the nanocarbon material can be bonded to the powdered metalby shaking the mill vessel in three dimensions. Accordingly, thenanocarbon material in the form of long fibers can be mixed with thepowdered metal in a desirable manner. In addition, in the method of thepresent invention, a sintered body, i.e., a nanocarbon/metal compositematerial can be obtained by sintering the uniformly mixed mixture of apowdered metal and nanocarbon material. Since the nanocarbon material isuniformly mixed with the powdered metal, a nanocarbon/metal compositematerial which has a large strength and a large thermal conductivity canbe manufactured.

The nanocarbon material that is prepared in the aforementionedpreparatory step is preferably a nanocarbon material that has beendispersed by ultrasound in advance. If a nanocarbon material that hasthus been dispersed by ultrasound is used, a nanocarbon/metal compositematerial in which the nanocarbon material is more favorably dispersedcan be manufactured.

BRIEF DESCRIPTION OF THE DRAWINGS

Several preferred embodiments of the present invention will be describedin detail below with reference to the attached figures, wherein:

FIGS. 1A through 1C show the method for mixing a powdered metal andnanocarbon material in accordance with the present invention;

FIG. 2 shows the mixture obtained by the mixing method shown in FIG. 1;

FIG. 3 shows the principle of a plasma sintering apparatus that is usedto perform a sintering treatment on the mixture shown in FIG. 2;

FIG. 4 is a graph showing the relationship between the sintering timeand sintering temperature in the sintering treatment;

FIG. 5 is a graph showing the relationship between the amount of carbonnanofibers added and the maximum tensile stress in samples 1 through 5;and

FIG. 6 shows a graph comparing the maximum tensile stress between sample6 and sample 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The method for mixing a powdered metal and a nanocarbon material inaccordance with the present invention will be described with referenceto FIGS. 1A through 1C.

As is shown in FIG. 1A, which shows the dispersion treatment step, ananocarbon material 13 is placed in a vessel 12 filled with an acetonesolution 11, and the vessel is placed on an ultrasonic vibratingapparatus 14. Next, the acetone solution 11 is shaken by the action ofthe ultrasonic vibrating apparatus 14. As a result, the nanocarbonmaterial 13 is dispersed. The nanocarbon material 13 is then removedfrom the vessel 12 and dried.

As is shown in FIG. 1B, no balls are placed in the metal mill vessel 15.Then, a specified amount of a powdered metal (e.g., powdered aluminum)16 and the nanocarbon material 17 dispersed in FIG. 1A are placed in theempty metal mill vessel 15.

As is shown in FIG. 1C, a cover 19 is placed on the mill vessel 15.Then, this mill vessel 15 is shaken in three dimensions. As a result, amixture of the powdered metal 16 and nanocarbon material 17 can beobtained.

The nanocarbon material 13 that has been dispersed by ultrasonicvibration in FIG. 1A may be dried while the powdered metal 16 is mixed.In this case, a mixed powder of the nanocarbon material 13 and powderedmetal 16 is placed in the metal mill vessel 15 shown in FIG. 1B.

Furthermore, the apparatus that shakes the mill vessel 15 in threedimensions in FIG. 1C is preferably a ball mill that has an agitatingaction in three dimensions, such as a tri-axial shaking ball mill,planetary ball mill, or the like.

As is shown in FIG. 2, the mixture of a powdered metal and nanocarbonmaterial obtained in FIG. 1C is a mixture 18 having a configuration inwhich the surfaces of a simple powdered metal 16 are coated with a finenanocarbon material 17.

The nanocarbon material 17 consisted of long fibers, and no signs ofcutting were observed.

Specifically, since only the powdered metal 16 and nanocarbon material17 were placed in the mill vessel, without any balls being placed inthis vessel, and the mill vessel was then shaken, no large cutting forcewas applied to the nanocarbon material 17. Accordingly, it appears thatit was possible to coat the powdered metal 16 with the long-fibernanocarbon material 17 in a substantially uniform manner.

Furthermore, it is desirable to perform a sintering treatment on themixture 18 shown in FIG. 2. The principle of a plasma sinteringapparatus that is suitable for this sintering treatment will bedescribed next.

As is shown in FIG. 3, the plasma sintering apparatus 20 consists of alower base 21; a lower spacer 22 that is mounted on this lower base 21;a lower punch 23 that extends upward from this lower spacer 22; acylindrical die 24 that is fitted into this lower punch 23; an upperpunch 25 that is centered on this die 24 and disposed symmetrically withrespect to the aforementioned lower punch 23; an upper spacer 26 whichretains this upper punch 25 from above; a lifting member 27 from whichthe upper spacer 26 is suspended; a vacuum chamber 28 which surroundsthe lower spacer 22, lower punch 23, die 24, upper punch 25, and upperspacer 26; a vacuum pump 29 which is attached to this vacuum chamber 28,and which evacuates the interior of the vacuum chamber 28; and a pulsepower supply 30 which is electrically connected to the lower spacer 22and upper spacer 26.

The lower spacer 22, lower punch 23, die 24, upper punch 25, and upperspacer 26 are all parts that are made of graphite and possess electricalconductivity. Accordingly, when a pulse current is supplied to the lowerspacer 22 and upper spacer 26 by the pulse power supply 30, plasma isgenerated between the lower punch 23 and upper punch 25.

The die 24 is filled with the mixture 18 (FIG. 2), and pulse powering isinitiated while the mixture is compressed between the lower punch 23 andupper punch 25 by pushing the upper punch 25 downward. Consequently,plasma is generated, and the mixture 18 can be heated by the high heatof the plasma. Graphite will burn in the atmosphere, but there is nooxygen in a vacuum, and hence no danger of burning there.

Since the plasma sintering apparatus 20 is capable of rapid heating, thetreatment time is short, which is advantageous from the standpoint ofincreasing productivity. Various types of sintering apparatuses havebeen adapted for practical use, and the type of apparatus used isarbitrary.

In the sintering treatment, the relationship between the treatmenttemperature and treatment time is important. One example of thetemperature curve used to determine this relationship will be describednext.

As is shown in FIG. 4, the temperature is elevated from room temperatureto 530° C. over a period of 6 minutes (360 seconds). The temperature isthen elevated to 570° C. over a period of 2 minutes, and is subsequentlyelevated to 580° C. over a period of 1 minute. The temperature is thenmaintained at 580° C. for 10 minutes. Following this period oftemperature maintenance, the supply of power is stopped, and the plasmasintering apparatus 20 shown in FIG. 3 is allowed to cool naturally inits entirety. This type of cooling is called oven cooling, and it ischaracterized by an extremely low cooling rate.

This type of temperature curve is merely an example. Specifically, thiscurve may be appropriately established on the basis of the metalmaterial that is prepared.

The experiments described below were performed in order to confirm theeffect of the manufacturing method of the present invention describedabove.

EXAMPLES

Examples of the present invention will be described below, but thepresent invention is not limited to these examples.

Preparation:

Ball mill: TKMAC-1200L manufactured by Topologic Systems

Metal mill vessel: internal diameter 55 mm, length 60 mm.

Capacity: approximately 140 mL, material: SUS 304.

Powdered metal: powdered aluminum having a mean particle size of 45 μm.Bulk density: 2.96 g/cm³, melting point: 660° C.

Nanocarbon material: carbon nanofibers having a maximum fiber diameterof 150 nm and a bulk density of 0.04 g/cm³. However, these carbonnanofibers were not dispersed by ultrasound.

Charging into Mill Vessel:

The total mass of powdered aluminum (powdered Al) and carbon nanofibers(CNF) was set at 20.0 g. These materials were placed in the mill vesselso that the amount of carbon nanofibers was 0 mass %, 0.5 mass %, 1.0mass %, 2.0 mass %, or 5.0 mass %, and the remainder was powderedaluminum. The concrete masses are shown in the following table. TABLE 1Sample Powdered No. Total CNF Ratio Aluminum CNF 1 20.0 g 0.0 mass %20.0 g 0.0 g 2 20.0 g 0.5 mass % 19.9 g 0.1 g 3 20.0 g 1.0 mass % 19.8 g0.2 g 4 20.0 g 2.0 mass % 19.6 g 0.4 g 5 20.0 g 5.0 mass % 19.0 g 1.0 gMixing of Powdered Metal and Nanocarbon Material:

The mill vessel was placed in the aforementioned ball mill and wasshaken for 5 hours at 800 rpm.

Sintering:

The mixture thus obtained was set in the plasma sintering apparatus 20shown in FIG. 3, and was sintered while being molded under the followingconditions.

Degree of vacuum: 5 Pa

Pressurization: 60 MPa

Heating curve: according to heating curve shown in FIG. 4

Tensile Test:

The sintered body thus obtained was treated by a rolling method at 300°C. A tensile test piece was manufactured from the rolled material thusobtained, this test piece was placed in a tensile tester, and themaximum tensile stress was determined. The results are shown in thefollowing table. TABLE 2 Sample Maximum Tensile No. CNF Ratio Stress 10.0 mass %  98 N/mm² 2 0.5 mass % 126 N/mm² 3 1.0 mass % 128 N/mm² 4 2.0mass % 122 N/mm² 5 5.0 mass % 105 N/mm²

FIG. 5 is a graph showing the relationship between the amount of carbonnanofibers added and the maximum tensile stress. The graph shows thenumerical values in the above table in the form of a graph. Sample 1contains no CNF. Samples 2 through 5 contain CNF. It was confirmed thatthe mechanical strength can be increased by mixing CNF.

Specifically, it can be said that a nanocarbon/metal composite materialhaving a large strength, as confirmed in samples 2 through 5, can bemanufactured by performing a preparatory step in which a ball mill, aspecified amount of a powdered metal, and a specified amount of ananocarbon material are prepared; a step in which the aforementionedpowdered metal and nanocarbon material are placed in an empty metal millvessel containing no balls; a step in which a mixture comprising theaforementioned powdered metal coated with the nanocarbon material isobtained by shaking the mill vessel in three dimensions using theaforementioned ball mill; and a step in which the aforementioned mixtureis molded and sintered to obtain a sintered body.

Next, a test investigating the effect of a dispersion treatment inimproving the effect of the present invention was additionallyperformed.

Dispersion Treatment:

Solution: acetone solution

Vibration frequency: 28 kHz

Treatment time: approximately 20 minutes

The subsequent preparation, charging into the mill vessel, mixing,sintering, and tensile testing were the same as in the case of samples 1through 5; accordingly, a description is omitted here.

The maximum tensile stress measured for sample No. 6 was as shown in thefollowing table. Sample 3, which showed the best results among samples 1through 5, is also shown for comparison. TABLE 3 Sample No. MaximumTensile Stress 6 133 N/mm² 3 128 N/mm²

FIG. 6 is a graph showing a comparison of sample 6 and sample 3according to the present invention. In both sample 6 and sample 3, theamount of CNF added is 1 mass %, and the two samples differ only interms of the presence or absence of a dispersion treatment, with sample6 being subjected to a dispersion treatment by ultrasound, and sample 3not being subjected to any dispersion treatment. The maximum tensilestress of sample 3 is 128 N/mm², and the maximum tensile stress ofsample 6 is 133 N/mm²; consequently, according to the calculation of133÷128=1.04, a 4% increase in strength is seen as an effect of thedispersion treatment.

Furthermore, although acetone is ideal as the solvent used in thedispersion treatment, some other similar solvent may also be used.

Moreover, in the step in which the mixture is molded and sintered toobtain a sintered body, it would also be possible to perform molding andsintering in series by manufacturing a molded body using a powderpressing method, and then transferring this molded body to a sinteringapparatus and performing a sintering process, besides performing moldingand sintering simultaneously in parallel as in the present example.

Obviously, various minor changes and modifications of the presentinvention are possible in the light of the above teaching. It istherefore to be understood that within the scope of the appended claimsthe invention may be practiced-otherwise than as specifically described.

1. A method for mixing a powdered metal and a nanocarbon material, saidmethod comprising the steps of: preparing a ball mill, a specifiedamount of a powdered metal, and a specified amount of a nanocarbonmaterial; placing said powdered metal and nanocarbon material in theempty metal mill vessel containing no balls; and obtaining a mixture inwhich said powdered metal is coated with the nanocarbon material byshaking said mill vessel in three dimensions using said ball mill. 2.The method of claim 1, wherein the nanocarbon material prepared in saidpreparatory step comprises a nanocarbon material dispersed in advance byultrasound.
 3. A method for manufacturing a nanocarbon/metal compositematerial, said method comprising the steps of: preparing a ball mill, aspecified amount of a powdered metal, and a specified amount of ananocarbon material; placing said powdered metal and nanocarbon materialin the empty metal mill vessel containing no balls; obtaining a mixturein which said powdered metal is coated with the nanocarbon material byshaking said mill vessel in three dimensions using said ball mill; andmolding and sintering said mixture to obtain a sintered body.
 4. Themethod of claim 3, wherein the nanocarbon material prepared in saidpreparatory step comprises a nanocarbon material dispersed in advance byultrasound.